Makefile: silence perl/PM.stamp recipe

Every time we run "make", we update perl/PM.stamp, which contains a list of all of the perl module files (if it's updated, we need to rebuild perl/perl.mak, since the Makefile will not otherwise know about the new files). This means that every time "make" is run, we see: GEN perl/PM.stamp in the output, even though it is not likely to have changed. Let's make this recipe completely silent, as we do for other auto-generated dependency files (e.g., GIT-CFLAGS). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>